While TNT-based explosives have been known for many years, in the military field of cast explosives for shaped charges and the like they suffer from several deficiencies. Such explosives have been subject to detonation in response to impact, flame, heat, and static electric loads. In addition, TNT melts at about 80.degree. C. and when cast it is subject to a dramatic volume change in the cool down to ambient temperature. TNT will exhibit a density at 80.degree. C. of 1.44 grams per cc. At ambient that density increases to 1.65 grams per cc. The volume decrease resulting is about 12%. Therefore, normal procedures use an overcasting process with "risers" which require then a machining of the cast explosive and usually discarding of the excess.
TNT is in wide use as an explosive and as an explosive base because it is cheap to produce, even in large quantities. TNT can be produced with low cost, low shear mixers that are easy to install and automate.
When TNT is mixed with other explosives, other deficiencies are exhibited. There often is the absence of a predictable response because the explosive mixture itself is nonhomogeneous. Explosive grains have been known to be subject to sedimentation. There also has been exhibited an absence of an explosive/lens bond which grossly affects the reliability of armor penetrating shaped charge munitions. In addition, the military standards require that the explosive be reliable over a temperature range of -65.degree. to +165.degree. F. without exhibiting brittleness or without exuding oil or the like and that this reliability be established over long term storage. Finally, many TNT explosives can exhibit detonation in massive storage which precludes the use of low cost nonreveted storage, low cost transportation, minimal tactical response times and minimum vulnerability of personnel in military platforms.
These deficiencies were alleviated in part with plastic bonded explosive systems (PBX's ). These systems are obtained by the incorporation of particulate explosives in an inert elastomeric substrate which attenuates the rate of energy transfer to provide below detonation levels while achieving a reproducibility adequate for most munitions. These results are achieved at the cost of performance and increased process costs. The cost in performance reflects the parasitic effect of the inert binder and the high process costs are caused by the necessity of using high shear and high cost mixers capable of handling the high viscosity characteristics of PBX systems.
In U.S. Pat. No. 3,447,980 it was disclosed that a polyurethane prepolymer in combination with a curing agent such as hydrogenated castor oil when combined with liquid TNT and one or more other explosives could produce a castable explosive which would not shrink on cooling. However, in subsequent patents such as U.S. Pat. No. 4,284,442, this disclosure was characterized as being difficult to use in that it had a very short pot life before gelation and exhibited irreversible chemical curing of the polyurethane elastomer. The solidified composition then could not be re-melted and re-formed as a mixture. Other attempts to provide a castable TNT-based explosive are described, for example, in U.S. Pat. No. 4,012,245 wherein the additive is an organic polyisocyanate, a hydroxyl-terminated butadiene polymer liquid resin and an abietyl alcohol. Also, in U.S. Pat. No. 4,325,759 TNT and a polymer are dissolved in methyl ethyl ketone and then re-precipitated as granules which are then incorporated in molten TNT.
There are several known high explosives which are more efficient for certain purposes than TNT. These include cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), nitroguanidine (NQ), 3 nitro 1,2,4 triazoyl 5-one (NTO) and ammonium picrate. For the most part these individual high explosives are too sensitive to be used alone. A common explosive is identified as Composition B (CompB) which typically consists of 60% RDX, 40% TNT, and added wax desensitizers. This composition, however, suffers from the above infirmities, and in addition, if the RDX crystals are dispersed in molten TNT, sedimentation may occur.